Technical Field
The present disclosure relates to a processing circuit for a digital microelectromechanical sensor, which includes two or more sensing structures and has a broad dynamic range. In addition, the present disclosure relates to a sensor that includes the aforementioned processing circuit.
Description of the Related Art
As is known, there are today available acoustic transducers such as, for example, the so-called MEMS (microelectromechanical systems) microphones, each of which comprises a sensing structure of a MEMS type, which is also known as “detection structure” and is designed to transduce acoustic pressure waves into an electrical quantity (for example, a capacitive variation), and a reading electronics, designed to carry out appropriate operations of processing of this electrical quantity, for supplying an electrical output signal, whether analog (for example, a voltage) or digital; in the latter case, the microphone is a digital microphone. For instance, with particular reference to electrical output signals of a digital type, MEMS microphones are known that supply signals of a so-called “PDM” (pulse-density modulation) type.
The electrical output signal is then made available, possibly after prior further processing by an electronic interface circuit, to an external electronic system, such as for example a microcontroller of an electronic apparatus that incorporates the MEMS microphone.
In the case of MEMS acoustic transducers of a capacitive type, each sensing structure comprises a fixed electrode and a mobile electrode, which is formed by a diaphragm or membrane and is arranged facing the fixed electrode, so that the fixed electrode and the mobile electrode form the plates of a sensing capacitor with variable capacitance. The sensing capacitor is typically connected to a charge pump, which performs the task of maintaining the charge present on the sensing capacitor itself constant.
More in particular, a perimetral portion of the mobile electrode is typically anchored to a substrate, whereas a central portion of the mobile electrode is free to move following upon incidence of an acoustic signal, i.e., a pressure wave. Consequently, at least a part of the mobile electrode is arranged in oscillation by the acoustic signal, with consequent variation of the capacitance of the sensing capacitor.
An example of a sensing structure of a MEMS microphone of a capacitive type is described in US Patent Publication No. 2010/0284553 filed in the name of the present applicant.
In general, the electrical performance of a MEMS microphone depends upon the mechanical characteristics of the sensing structure, and further upon the configuration of the acoustic chambers formed by the sensing structure; in this connection, the sensing structure forms a front chamber and a rear chamber, which face, respectively, the front face and the rear face (opposite to one another) of the mobile electrode and are traversed, in use, by the pressure waves that impinge upon the sensing structure.
From a more quantitative standpoint, it is possible to characterize a sensing structure in terms of sensitivity and dynamics, the latter quantity being also known as “dynamic range”.
The dynamic range indicates the sound-pressure levels (SPL) of the acoustic signals that may be correctly demodulated by the sensing structure. Consequently, the upper bound of the dynamic range indicates the sound-pressure level beyond which a saturation of the response of the sensing structure occurs, whereas the lower bound indicates the noise level, i.e., the sound-pressure level below which the acoustic signal is not detected.
The sensitivity is instead proportional to the ratio between the variation of the aforementioned electrical quantity (for example, the capacitance of the sensing capacitor) and the corresponding variation of the sound-pressure level.
This having been said, there are numerous applications in which there are used both a broad dynamic range, i.e., the possibility of detecting acoustic signals that have sound-pressure levels markedly different from one another, and a high sensitivity. Unfortunately, however, typically the sensing structures that have a high sensitivity are characterized also by narrow dynamic ranges, and vice versa. In addition, typically the sensing structures that have broad dynamic ranges are characterized by not particularly high signal-to-noise ratios (SNRs).
In this connection, U.S. Pat. No. 6,271,780 describes a solution for increasing the dynamic range, which envisages subjecting an analog input signal to two processing paths, each of which comprises a first, analog, portion and a second, digital, portion; further, each processing path is characterized by its own gain. The digital signals at output from the two processing paths are recombined to supply a resulting output signal. Before the two digital signals are recombined, they are subjected to operations of equalization for compensating for the differences present between the two processing paths, but for the different gains, in order to limit the distortions present on the resulting output signal.
The solution proposed in U.S. Pat. No. 6,271,780 is not free from problems, linked principally to the complexity of the processing chain, and thus to the dimensions of the area used for implementing this solution. In addition, this solution envisages that, starting from a single input signal, two intermediate signals are generated, which are then mixed to form an output signal.